Sec Form 4 Filing - Atlas Venture Fund X, L.P. @ Disc Medicine, Inc. - 2023-12-12

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Atlas Venture Fund X, L.P. 2. Issuer Name and Ticker or Trading Symbol Disc Medicine, Inc. [ IRON] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 12/12/2023 (Street) CAMBRIDGE, MA02139 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 12/12/2023 S 31,235 D $ 59.87 ( 1 ) 1,562,550 D ( 2 ) Common Stock 12/12/2023 S 20,823 D $ 59.87 ( 1 ) 861,401 I See footnote ( 3 ) Common Stock 12/12/2023 S 632 D $ 60.99 ( 4 ) 1,561,918 D ( 2 ) Common Stock 12/12/2023 S 422 D $ 60.99 ( 4 ) 860,979 I See footnote ( 3 ) Common Stock 12/12/2023 S 6,234 D $ 62.42 ( 5 ) 1,555,684 D ( 2 ) Common Stock 12/12/2023 S 4,155 D $ 62.42 ( 5 ) 856,824 I See footnote ( 3 ) Common Stock 12/12/2023 S 994 D $ 62.9 ( 6 ) 1,554,690 D ( 2 ) Common Stock 12/12/2023 S 665 D $ 62.9 ( 6 ) 856,159 I See footnote ( 3 ) Common Stock 12/12/2023 S 1,031 D $ 63.78 ( 7 ) 1,553,659 D ( 2 ) Common Stock 12/12/2023 S 688 D $ 63.78 ( 7 ) 855,471 I See footnote ( 3 ) Common Stock 12/12/2023 S 27,924 D $ 65.1 ( 8 ) 1,525,735 D ( 2 ) Common Stock 12/12/2023 S 18,615 D $ 65.1 ( 8 ) 836,856 I See footnote ( 3 ) Common Stock 12/12/2023 S 9,901 D $ 65.77 ( 9 ) 1,515,834 D ( 2 ) Common Stock 12/12/2023 S 6,599 D $ 65.77 ( 9 ) 830,257 I See footnote ( 3 ) Common Stock 12/13/2023 S 5,263 D $ 59.99 ( 10 ) 1,510,571 D ( 2 ) Common Stock 12/13/2023 S 3,508 D $ 59.99 ( 10 ) 826,749 I See footnote ( 3 ) Common Stock 12/13/2023 S 60 D $ 60.5 1,510,511 D ( 2 ) Common Stock 12/13/2023 S 40 D $ 60.5 826,709 I See footnote ( 3 ) Common Stock 12/14/2023 S 30,645 D $ 58.27 ( 11 ) 1,479,866 D ( 2 ) Common Stock 12/14/2023 S 20,429 D $ 58.27 ( 11 ) 806,280 I See footnote ( 3 ) Common Stock 12/14/2023 S 180 D $ 59.74 ( 12 ) 1,479,686 D ( 2 ) Common Stock 12/14/2023 S 120 D $ 59.74 ( 12 ) 806,160 I See footnote ( 3 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Atlas Venture Fund X, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X ATLAS VENTURE ASSOCIATES X, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates X, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Opportunity Fund I, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates Opportunity I, L.P. 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA02139 X Atlas Venture Associates Opportunity I, LLC 300 TECHNOLOGY SQUARE, 8TH FLOOR CAMBRIDGE, MA X Signatures Atlas Venture Fund X, L.P., By: Atlas Venture Associates X, L.P., its general partner, By: Atlas Venture Associates X, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 12/14/2023 Signature of Reporting Person Date Atlas Venture Associates X, L.P., By: Atlas Venture Associates X, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 12/14/2023 Signature of Reporting Person Date Atlas Venture Associates X, LLC By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 12/14/2023 Signature of Reporting Person Date Atlas Venture Opportunity Fund I, L.P., By: Atlas Venture Associates Opportunity I, L.P., its general partner, By: Atlas Venture Associates Opportunity I, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 12/14/2023 Signature of Reporting Person Date Atlas Venture Associates Opportunity I, L.P., By: Atlas Venture Associates Opportunity I, LLC, its general partner, By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 12/14/2023 Signature of Reporting Person Date Atlas Venture Associates Opportunity I, LLC By: Ommer Chohan, Chief Financial Officer /s/ Ommer Chohan 12/14/2023 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $59.50 to $60.497 inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in footnotes (1), (4), (5), (6), (7), (8), (9), (10), (11) and (12). ( 2 )These shares are held directly by Atlas Venture Fund X, L.P. ("Atlas X"). Atlas Venture Associates X, L.P. ("Associates X") is the general partner of Atlas X. Atlas Venture Associates X, LLC ("Associates X LLC") is the general partner of Associates X. Each of Associates X and Associates X LLC disclaims Section 16 beneficial ownership of the securities held by Atlas X, except to the extent of its pecuniary interest therein, if any. ( 3 )These shares are held directly by Atlas Venture Opportunity Fund I, L.P. ("Atlas Opportunity I"). Atlas Venture Associates Opportunity I, L.P. ("Associates I") is the general partner of Atlas Opportunity I. Atlas Venture Associates Opportunity I, LLC ("Associates I LLC") is the general partner of Associates I. Each of Associates I and Associates I LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Opportunity I, except to the extent of its pecuniary interest therein, if any. ( 4 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $60.51 to $61.50 inclusive. ( 5 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $61.5297 to $62.5175 inclusive. ( 6 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $62.6436 to $63.3256 inclusive. ( 7 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $63.70 to $64.67 inclusive. ( 8 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $64.7499 to $65.735 inclusive. ( 9 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $65.745 to $66.32 inclusive. ( 10 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $59.50 to $60.4381 inclusive. ( 11 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $58.25 to $58.65 inclusive. ( 12 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $59.53 to $60.17 inclusive.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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